BN rats do not reject F344 brain allografts even after systemic sensitization

Embryonic brain tissue allografts under many circumstances survive transplantation into the brain. It is generally believed that such grafts will not survive if the host animal is systemically sensitized, by skin grafting or other means, to major histocompatibility complex (MHC) antigens of the donor animal. We have found that F344 brain grafts survive in BN hosts even when the host is systemically sensitized to F344 tissue. Embryonic cerebral neocortex from F344 donors was transplanted into BN host rats (n = 95). Subsequently, the host rats were systemically sensitized with donor skin (n = 25), brain tissue (n = 41), or spleen cells (n = 6) and compared with a control group of rats consisting of allografts with no sensitization or sham procedures (n = 23). Rejection of the transplants in BN rat hosts was not provoked by any of the sensitization methods tested. Minor immunological responses that did not result in rejection were, however, present in many host animals. We did not observe infiltration of W3/13+ T cells and OX8+ cytotoxic lymphocytes in any of the groups. Nevertheless, substantial infiltrations of OX6+ antigen-presenting cells and W3/25+ helper T cells were present. There was also an extensive enhancement of MHC class I immunoreactivity in parts of the grafted tissue developing within the third ventricle, but not for the same type of graft in the lateral ventricle. This increase of MHC class I expression was not accompanied by infiltration of cytotoxic T cells. Our findings thus suggest that neural graft rejection depends on general genetic susceptibility to immune reactions, particularly experimental allergic encephalomyelitis and not only on disparity between donor and host antigens encoded by the MHC. Moreover, enhancement of MHC class I and class II expression within transplanted tissue does not predict graft rejection.     

Immunohistochemical analysis on the rejection of xenogeneic brain grafts

The central nervous system (CNS) of mammals has long been thought of as an immunologically privileged site. However, this concept is now changing because the rejection of histo-incompatible neural grafts has been frequently observed in the CNS. In neural transplantation used as therapy for some human neurodegenerative diseases, it is important to determine which factors are related to brain graft rejection. In this study, we examined immunological reactions in brains that had received isogeneic (rat to rat) and xenogeneic (mouse to rat) neural transplants. In the immunohistochemical analysis, antibodies against T cell receptor αβ (R73), macrophage and microglia (0X42), MHC class II antigens (0X6), CD4 (W3/25), CD8 (0X8), NK cell (3.2.3), B cell (RLN-9D3), T cell receptor (TCR) Vβ8.2 (R78), TCR Vβ8.5 (B73) and TCR Vβl0 (G101) were used. At the early stage of both isogeneic and xenogeneic transplantation, a nonspecific inflammatory reaction characterized by macrophage infiltration was observed along the needle track which was produced by the grafting procedure. From the day 7 stage onwards, the non-specific inflammatory reaction was replaced by the specific immune reactions of T cell infiltration, neovascularization and necrosis of xenogeneic grafts. Marked T cell infiltration was detected in the lesions, whereas NK and B cells were not. Quantitative analysis of T cell subsets revealed that both CD4+ and CD8+ T cells were found in the xenogeneic transplants. Microglia became activated and strongly expressed MHC class II antigens at the time of graft rejection. Isogeneic transplants, in contrast, showed no histological characteristics of rejection, and numerous dopaminergic neurons with several neurites were observed in the grafts. Based on these findings, we concluded that T cells are the principal effectors in the rejection of xenogeneic neural grafts, and that activated microglia may have some role in presenting antigens to the infiltrating T cells during the rejection process.     

Effect of intrathymic injection of allogene antigen on immune response to sciatic nerve transplantation in allogenic mice

BACKGROUND: The latest researches demonstrate that intrathymic injection of MHC antigen which reaches a certain dosage (2 mg, i.e., 4 × 108 cell extraction) can induce immunologic tolerance under non-antilymphocyte serum condition. OBJECTIVE: To investigate the effect of intrathymic injection of allogene antigen on survival and function of sciatic nerve in allogenic mice. DESIGN: Randomized controlled animal study. SETTING: The 4th Affiliated Hosptial of Harbin Medical University. MATERIALS: A total of 32 male donor C57BL/6(H-2b) mice of 4–8 weeks old and weighing 18–22 g and 44 female receptor Balb/c(H-2d) mice of 4–8 weeks old and weighing 18–22 g were selected from Heilongjing Veterinary Institution. The animal experiment had got confirmed consent from local ethic committee. METHODS: The experiment was carried out in the Laboratory (Provincial Key Laboratory) of the Fourth Hospital, Harbin Medical University from June 2006 to May 2007. C57BL/6(H-2b) mice were anesthetized to extract MHC (H-2b) antigen from splenic cells and sciatic nerves. Allogenous nerve transplantation group: Mice were given intrathymic injection of 100 μL saline; two weeks later, frozen sciatic nerves of donor mice were transplanted. Immunosuppressive agent group: Mice were given intrathymic injection of 100 μL saline; two weeks later, fresh sciatic nerves of donor mice were transplanted. At three days before transplantation, 10 mg/kg per day cyclosporin A was intraperitoneally injected once a day till mice were sacrificed. MHC (H-2b) antigen injection group: Mice were given intrathymic injection of MHC (H-2b) antigen from C57BL/6(H-2b) donor mice; two weeks later, fresh sciatic nerves of donor mice were transplanted. Autogenous nerve transplantation group: Mice were given intrathymic injection of 100 μL saline; two weeks later, fresh sciatic nerves were transplanted. MAIN OUTCOME MEASURES: ① Three weeks later, transplanted part of exposured sciatic nerve was used to measure the motor nerve conduction velocity. ② Transplanted nerve was stained with histochemical staining and observed light microscope and electron microscope. ③ Mice received mixed lymphocyte culture and delayed-typed hypersensitiveness to observe absorbency and measure depth of soles. RESULTS: All 76 mice were involved in the final analysis. ① Motor nerve conduction velocity: The nerve recovery in MHC (H-2b) antigen injection group was higher than that in allogenous nerve transplantation group, equal to immunosuppressive agent group and lower than autogenous nerve transplantation group. There were significant differences among them (P < 0.05). ② Histological changes of transplanted nerve: Light and electron microscopes demonstrated that there were a lot of regenerative nerve fibers in autogenous nerve transplantation group, immunosuppressive agent group and MHC (H-2b) antigen injection group, and all nerve fibers passed grafts. ③ Immunological examination: There was no significant difference in mixed lymphocyte culture among allogenous nerve transplantation group, autogenous nerve transplantation group and MHC (H-2b) antigen injection group (P < 0.05). Depth of soles in other groups was deeper than that in the MHC (H-2b) antigen injection group, and there was significant difference (P < 0.05); that was to say, delayed-typed hypersensitiveness was remarkable. CONCLUSION: The intrathymic injection of allogene MHC antigen may induce specific immune tolerance to allogenous sciatic nerve transplantation and promote nerve survival and functional recovery.     

Immunological reactions to neural grafts in the central nervous system

Immunological rejection is a lasting, although highly variable, threat to allo- and xenogeneic neural tissue grafted to the CNS of rodents, monkeys and man. One major determinant for rejection of intracerebral CNS grafts appears to be induction of major histocompatibility complex (MHC) antigens on the donor CNS cells. We have previously examined the cellular immune response against neural mouse xenografts undergoing rejection in the adult rat brain. In this study we focus on the astro- and microglial reactions within and around the graft, and the potential of individual host rat and donor mouse brain cells to express MHC antigens. Previous light microscopical observations of expression of rat MHC antigen class I by endothelial cells, microglial cells, and invading leukocytes were extended to the ultrastructural level and found to include a few astrocytes. Rat and mouse MHC antigen class II was only detected on leukocytes and activated microglial cells. The findings imply that within grafts of brain or spinal cord tissue donor astrocytes, microglial cells and endothelial cells can be induced to act as target cells for class I specific host T cytotoxic cells, while only (graft and host) microglial cells can be induced to express MHC antigen class II and present antigen to sensitized (and possibly also resting) host T helper cells.     

Expression of H-2 antigen on oligodendrocytes is induced by soluble factors from concanavalin A activated T cells

It has been shown that soluble factors from activated T cells, or interferon alone, enhance the expression of major histocompatibility complex (MHC) antigens in several cell types. In this study we have demonstrated, by means of indirect immunofluorescence and radioimmunoassay, that the expression of mouse MHC class I antigen (H-2) on isolated mouse oligodendrocytes is induced by soluble factors from concanavalin A activated T cells. Autoradiographic studies indicate that this induction of H-2 expression is not accompanied by proliferation of oligodendrocytes.     

MHC antigen expression on bulk isolated macrophage-microglia from newborn mouse brain: induction of Ia antigen expression by gamma-interferon

Macrophage-microglia were isolated from primary mixed brain cell cultures of normal newborn mice. They were successfully maintained in vitro for at least 8 weeks. Purity of the cultures was 97-100%, as determined by endocytosis of latex beads, non-specific staining through Fc receptors, EA and EAC rosette formation. These cells were non-specific esterase-positive, but peroxidase-negative. Electron-microscope observations revealed morphological similarities to mature macrophages. Isolated macrophage-microglia seldom incorporated [3H]thymidine in vitro. By means of 51Cr release assay, using monoclonal antibodies against mouse major histocompatibility complex (MHC) antigens and complement, we detected class I MHC (H-2) antigen on unstimulated macrophage-microglia, and both class I and class II (Ia) antigens on gamma-interferon-treated cells. These observations suggest possible immunoregulatory functions of macrophage-microglia in the central nervous system, as is characteristic of other cells of monocyte lineage.     

Neuronal progenitors identified by their inability to express class I histocompatibility antigens in response to interferon-γ

Interferon-γ (IFN-γ) can induce class I major histocompatibility complex (MHC) antigen (H-2) expression on virtually all neuroepithelial cells isolated from embryonic day 9 (E9) mice. However, a subpopulation of cells become refractory to H-2 induction (H-2l^?) by E10 and the percentage of H-2 noninducible cells increases during development. Cell sorting, by flow cytometry or magnetic bead immunoselection, has shown that H-2l^? cells give rise exclusively to neuronal cells, and by E12, the majority of the neuronal progenitors are found within this population. It has also been found that 98% of the H-2l^? also express the neuron-associated marker, A2B5. Cells of the glial cell lineage retain the ability to express class I antigens throughout development. From these studies, it is clear that the neuroepithelium contains cells committed to the neuronal cell lineage as early as E10 in the mouse. Copyright © 1994 Wiley-Liss, Inc.     

Expression of Ia antigen on vascular endothelial cells in mouse cerebral tissue grafted into the third ventricle of rat brain

The mechanisms of the immunological rejection after xenogeneic neural transplantation were investigated with special reference to the expression of class II major histocompatibility complex (MHC) antigen (Ia antigen) on the grafted tissue. Tissue from a newborn mouse cerebral cortex was transplanted into the third ventricle of a 4-week-old rat brain. Infiltration of cytotoxic T-cells into the grafted tissue was investigated immunohistochemically by using a monoclonal antibody (OX-8). The infiltration began 8 days after transplantation and continued until about 4 weeks when the tissue was completely rejected. The expression of Ia antigen was also investigated immunohistochemically. The Ia antigen was first detected in the grafted tissue at 6 days after transplantation. The Ia antigen was considered to be expressed on the vascular endothelial cells judging from the staining patterns and the location of India ink which was perfused from the host's left cardiac ventricle. The perfusion experiments with India ink also revealed that blood was supplied to the grafted tissue from 5 days after transplantation. These results suggest that the expression of Ia antigen on the vascular endothelial cells renders the grafted tissues competent to initiate and participate in the immune reaction. The results also raise a possibility that the expression of Ia antigen is triggered by blood supplied from the host brain. In addition, the results indicate that the Ia-positive blood vessels do not originate in the host brain but are intrinsic to the grafted tissue.     

Expression of major histocompatibility complex antigens on inflammatory peripheral nerve lesions

The expression of major histocompatibility complex (MHC) antigens by cells of the rat peripheral nervous system (PNS) was studied using a model of peripheral nerve transplantation. Monoclonal antibodies to polymorphic determinants of MHC class I and class II (Ia) molecules were used to determine donor or recipient origin of MHC antigen-bearing cells in nerve allografts. The expression of class I and class II antigens by PNS parenchymal cells was modified during varying alloimmune conditions. Baseline, constitutive expression of class I antigens on endothelial and perivascular cells and class II antigens on interstitial cells were identified. Decreased MHC antigen expression was noted following in vitro culture of nerve allografts prior to implantation. After transplantation, enhanced donor-derived MHC antigen expression was demonstrated by both cultured and untreated allograft endothelial, perivascular and interstitial cells in a pattern which was distinct from isografts. This data supports a concept of perivascular monocytic and/or parenchymal cell (Schwann cell or resident macrophage-like cell) activity as the resident antigen-presenting cell for PNS immune processes.     

Effects of the major histocompatibility complex loci and T-cell receptor beta-chain repertoire on Theiler's virus-induced demyelinating disease

We have investigated the potential effects of H-2 and T-cell receptor (TCR) V beta family genes on induction of T-cell immunity and susceptibility to virally induced demyelinating disease by using BALB.S (H-2K(s)A(s)D(s)) and BALB.S 3 R (H-2K(s)A(s)D(d)/L(d)) mice. These parameters were compared with those of highly susceptible SJL/J (H-2K(s)A(s)D(s)) mice that contain only one-half of TCR V beta family genes compared with the above-mentioned strains. Our results demonstrate that BALB.S but not BALB.S 3 R mice are susceptible similar to SJL/J mice. Although the level of CD4(+) T-cell infiltration to the CNS was elevated in susceptible mice, virus-specific immune responses restricted with H-2(s) were similar in these mice. No preferential use of V beta families associated with differences in the major histocompatibility complex (MHC) components was apparent. However, the pattern and sequence of CDR 3 distribution shows T-cell clonal accumulation in the CNS associated with the H-2 components. Further anti-CD8 antibody treatment of resistant BALB.S 3 R mice abrogated resistance to demyelinating disease, indicating that CD8(+) T cells restricted with H-2D(d)/L(d) are most likely to exert resistance in BALB.S 3 R mice. These studies indicated that TCR V beta and MHC class II genes are the secondary to a particular MHC class I gene expression in susceptibility to virally induced demyelinating disease.     

Differential expression of class I and class II major histocompatibility complex antigen in early postnatal rats

Cells expressing major histocompatibility complex (MHC) antigens are rarely found in normal mature brains, but cells resembling microglia can be induced to express these antigens following the onset of neural degeneration. In young rats, these cells show spontaneous expression of class I MHC antigens, which is further enhanced in the superior colliculus by the degeneration resulting from eye removal. By contrast, class II MHC antigen expression does not occur spontaneously and can only be induced by eye removal when the lesion is performed after the first postnatal week, when the optic tract begins to myelinate. We suggest that different signals are responsible for induction of class I and of class II MHC antigen expression.     

An immunoelectron microscopical study of the expression of class II major histocompatibility complex during chronic relapsing experimental allergic encephalomyelitis in Biozzi AB/H mice

Immunoelectron microscopical techniques have been used to study class II major histocompatibility complex (MHC) expression by cells in the spinal cords of Biozzi AB/H mice with chronic relapsing experimental allergic encephalomyelitis. Throughout the course of disease both astrocytes and endothelia failed to express significant levels of class II MHC antigens. The major central nervous system resident cell types found to express class II MHC antigens were the perivascular microglia, with infiltrating macrophages and some lymphocytes being strongly positive.     

Transplantation of a temperature-sensitive, nerve growth factor-secreting, neuroblastoma cell line into adult rats with fimbria-fornix lesions rescues cholinergic septal neurons

The HT4 cell line was derived from infection of a mouse neuroblastoma cell line with a retrovirus that encoded the temperature-sensitive (ts) mutant of SV40 large T antigen. At nonpermissive temperature, HT4 cells differentiated with neuronal morphology, expressed neuronal antigens, synthesized nerve growth factor (NGF) mRNA, and secreted biologically active NGF in vitro. We sought to establish whether transplanted HT4 cells expressed class I major histocompatibility complex (MHC) antigens, a partial requirement for recognition by cytotoxic T lymphocytes (CTL), and thus be susceptible to xenograft rejection. Differentiated HT4 cells expressed marginally detectable levels of class I MHC antigens, but demonstrated higher levels of class I MHC expression after treatment with interferon-gamma. However, HT4 cells were resistant to direct lysis by perforin, the pore-forming protein of CTLs, and thus may have potential use in xenograft experiments. To address whether HT4 cells secrete NGF in vivo, HT4 cells were transplanted into adults rats with unilateral fimbria-fornix transections. A ts cell line derived from P4 cerebellum, BT1, that does not differentiate with neuronal phenotype or synthesize NGF in vitro, was transplanted as a control. Six weeks posttransplant. HT4 cells had integrated into host CNS without forming tumors. In BT1 transplants, the number of medial septal acetylcholinesterase (AChE)-positive cells was reduced to 26-39% of the contralateral control side, depending on the rostrocaudal level. In HT4 transplants, the number of cholinergic septal neurons was 58-78% of the contralateral side. This percentage was significantly (P less than 0.005) greater than that seen with BT1 transplants, indicating that transplanted HT4 cells secrete NGF in vivo and rescue cholinergic septal neurons following fimbria-fornix transection.     

Survival of intrastriatal xenografts of ventral mesencephalic dopamine neurons from MHC-deficient mice to adult rats

Previous studies of neural xenografts have used immunosuppressive agents to prevent graft rejection. In the present study we have examined the survival of mouse dopamine neurons lacking either MHC class I or MHC class II molecules transplanted into rat brains and the host immune and inflammatory responses against the xenografts. Survival of neural grafts was immunocytochemically determined at 4 days, 2 weeks, and 6 weeks after transplantation by counting tyrosine hydroxylase (TH)-positive cells in the graft areas. In addition, the host immune and inflammatory responses against neural xenografts were evaluated by semiquantitatively rating MHC class I and class II antigen expression, accumulation of macrophages and activated microglia, and infiltration of CD4- and CD8-positive T-lymphocytes. For the negative controls, the mean number of TH-positive cells in rats that received wild-type mouse tissue progressively decreased at various time periods following transplantation. In contrast, intrastriatal grafting of either MHC class I or MHC class II antigen-depleted neural xenografts resulted in a prolonged survival and were comparable to cyclosporin A-treated rats that had received wild-type mouse tissue. These results indicate that genetically modified donor tissue lacking MHC molecules can be used to prevent neural xenograft rejection.     

Genetic regulation of class I major histocompatibility complex (MHC) antigen induction on astrocytes

Neural cells, including astrocytes, normally do not express detectable levels of class I major histocompatibility complex (MHC) molecules, unlike cells of most tissues. However, upon cultivation in vitro, astrocytes begin to express class I molecules, increasing with time after plating. This spontaneous expression was examined in the present study to characterize inducible expression on astrocytes among various strains of mice. Inducible expression, either as a consequence of cultivation or standard gamma-interferon treatment, differed markedly among the strains examined. Analysis of congenic strains on a C57BL/10 (B10) background showed that expression was controlled by genes within the MHC locus. Examination of additional congeneic animals with various recombinations within the MHC showed that high or low expression of MHC molecules correlates with the presence of particular MHC class I genes. In general, H-2a and H-2d class I products are expressed much higher on astrocytes than H-2b and H-2s products. This difference in expression is not seen on spleen cells indicating tissue specificity. Moreover, levels of expression at the cell surface are reflected by the steady-state level of RNA message within astrocytes of the different strains.     

Expression of major histocompatibility complex antigens in the brains of patients with progressive multifocal leukoencephalopathy.

Progressive multifocal leukoencephalopathy (PML) is caused by JC virus (JCV) infection of the central nervous system (CNS) in immunosuppressed patients. The immunopathogenesis of this chronic encephalitis is unknown. Because major histocompatibility (MHC) class I and class II antigens are important in modulating the immune response and viral clearance, we examined the tissue expression of MHC molecules in relation to CNS damage and presence of virus. By immunocytochemical staining, both MHC class I and class II antigens were expressed at high levels within PML lesions. Beta-2 microglobulin (beta-2m) was present on endothelial cells and JCV-infected oligodendroglia within the lesions. Also, many astrocytes with bizarre morphology expressed MHC class I antigens. In histologically normal regions of PML brains expression of beta-2m was noted only on endothelial cells. Expression of MHC class II also was focused within demyelinating lesions and was restricted to macrophages/microglia and occasional endothelial cells. When compared to other viral encephalitides (e.g. human immunodeficiency virus) these findings suggest that intra-CNS immune response to JCV is appropriate for antigenic presentation; however, the absence of responsive systemic T-cells may lead to chronic viral infection with progressive neuropathology.     

MHC antigen expression and cellular response in spontaneous and induced rejection of intracerebral neural xenografts in neonatal rats

Fetal mouse retinae transplanted to the mesencephalon of neonatal rats generally survive for prolonged periods of time without immune suppression suggesting that such grafts enjoy a degree of immunological privilege. A small, but consistent percentage of these transplants, however, ultimately undergo spontaneous rejection. In addition, rejection can be induced by (1) systemically sensitizing the host to the donor antigens by placing a mouse skin graft or (2) producing a local degenerative process adjacent to the graft by removing the host eye contralateral to the side of the retinal transplant. To elucidate the immunological events that underly spontaneous and induced rejection in this system, we examined the distribution of lymphocytes, astrocytes, microglia, and cells expressing major histocompatibility complex (MHC) antigens in unrejected grafts, in transplants showing spontaneous rejection, and in grafts undergoing induced rejection. In unrejected grafts, increased astrocytic and microglial staining was seen around the photoreceptor layer of the graft and at the graft-host interface, but no lymphocytes and only occasional cells expressing MHC antigens were detected. In contrast, spontaneously rejecting grafts showed widespread MHC, lymphocytic, astrocytic, and microglial immunoreactivity that extended well beyond the limits of the transplant into the surrounding host brain. Skin graft-induced rejection produced a temporally consistent, comparatively localized enhancement of astrocytic, microglial and MHC immunoreactivity and infiltration of lymphocytes. Four to five days after skin grafting, before neural graft rejection was detectable histologically, MHC immunoreactivity was demonstrated within the transplant coinciding with the presence of small numbers of lymphocytes and an increase in microglial staining. By 8 days, grafts had undergone profound necrosis. Intense astrocytosis, microglial staining, MHC immunoreactivity, and perivascular lymphocytic cuffing were present within the graft and at the graft-host interface. With longer survival times, several of these changes were also detected within the visual pathways, suggesting that the regions to which the graft projected were also involved, though in a delayed fashion. After eye removal, the temporal pattern of rejection was more protracted and considerably less uniform than that seen after skin grafting. At 7 days, prominent microglial, astrocytic, and MHC immunoreactivity was seen in the area of distribution of the host optic axons within the superior colliculus and to a lesser extent around the graft itself, however, no infiltration of lymphocytes was detected. With longer survival times, an increasing percentage of grafts showed signs of overt rejection with perivascular cuffing by lymphocytes; however, even at 21 days, a small number of grafts remained free of lymphocytic infiltration, despite the presence of intense MHC, astrocytic, and microglial staining. We conclude that the different rejection models studied may involve fundamentally different triggers of the host immune system, but that in each case MHC expression may be the precedent step to graft rejection.     

Peripheral nerve lesion produces increased levels of major histocompatibility complex antigens in the central nervous system

Proliferation of central nervous system (CNS) glia in response to peripheral nerve injury occurs without apparent participation of cells of the immune system. It is shown here that following transection of the rat facial nerve there is strongly elevated expression of class I, and to a lesser extent, class II antigens of the major histocompatibility complex (MHC) in the facial nucleus. It is demonstrated by double-immunofluorescence studies that the cells responsible for increased levels of MHC class I antigens are endogenous brain microglia. These findings emphasize the thought that microglia are immunocompetent cells, but, at the same time, raise the possibility for a non-immunological function of MHC antigens under conditions of neural regeneration.     

Immunohistochemical studies of adult human glial cells

Using immunohistochemical techniques, we examined major histocompatibility complex (MHC) antigen expression on astrocytes, oligodendrocytes, and macrophages-microglia derived from surgically resected tissue from young adults and maintained in dissociated cell cultures supplemented with either fetal calf or human AB serum. The majority of these cells in culture expressed class I MHC antigens. MHC class II expression was observed on only a restricted proportion of astrocytes either under basal or induction conditions (gamma-interferon, activated lymphocyte supernatants), on the majority of macrophages-microglia under inducing conditions, and not on oligodendrocytes. MHC class II expression on astrocytes in culture did not correlate with the extent of in situ gliosis or with in vitro cell morphology. MHC antigen expression was not detected in situ immunohistochemically. These data extend observations on the dissociation of in vivo and in vitro expression of MHC antigens on glial cells. The apparent greater expression of MHC class II antigens on macrophages-microglia compared to astrocytes raises the issue of the relative roles of each of these cell types in promoting immune reactivity under pathologic conditions.     

Allogeneic grafts of fetal dopamine neurons: immunological reactions following active and adoptive immunizations

To define the importance of adoptive sensitization and duration of graft residence on transplant alloimmunization, behavioral and histochemical parameters were examined in unilaterally 6-OHDA-lesioned F344 rat hosts which received fetal ventral mesencephalic (VM) grafts from Wistar-Furth (WF) donors. In all animals which showed increased rotations after alloimmunization, increased numbers of T cell receptor (TcR) positive, CD8⁺ lymphocytes were detected in the grafts. In addition, an increased density of class I MHC antigens was seen in the graft and in the adjacent host brain. Lesser numbers of CD4⁺, CD11b⁺, and MHCII⁺ positive elements were also seen. Perivascular cuffing was often found in actively immunized animals. An increase in TcR⁺ and MHC class I⁺ elements was also seen in animals only adoptively immunized. The tyrosine hydroxylase positive graft area was also markedly reduced in actively immunized animals and the extent of reduction correlated with the number of cells used for immunization. These studies indicate that established allografts can evade rejection as long as host lymphocytes are not activated against graft alloantigens. In addition, increasing graft residence time in the host and adoptive immunization render the graft more susceptible to subsequent rejection.